1. Field of the Invention
The present invention generally relates to metrology systems and methods for high aspect ratio and large lateral dimension structures.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
As stated in “Semiconductor metrology beyond 22 nm: 3D memory metrology,” Arceo et al., Solid State Technology (online edition), Feb. 16, 2012, which is incorporated by reference as if fully set forth herein, summarizing state-of-the-art metrology choices for high aspect ratio (HAR) structures, “Many optical techniques, especially those that operate off-axis near the critical angle, suffer from a very small fraction of the interrogating light reaching the feature bottom, and reflect upwards to the detector. Thus, in most cases, the various metrology techniques in their present forms will suffer low signal-to-noise ratios (SNRs) on such features.” In fact, no fast throughput and reliable metrology exists for measuring HAR structures, which will continue to be problematic as aspect ratios increase to 30:1, 100:1 and beyond. While techniques such as critical dimension (CD) small angle X-ray scatterometry (CD-SAXS), normal incidence reflectometry, and scatterometry are being explored and are also mentioned in the above reference, no adequate solution has been found yet. Meanwhile, the ability to measure CDs, defining the shapes of holes and trenches, is critical in achieving desired yields and high performance levels of devices.
Other challenges that exist in semiconductor device metrology are related to targets that have large lateral scales, e.g., on the order of 1 micron or larger. Since optical CD metrologies are predominantly designed for periodic targets, relatively large pitch targets can generate multiple diffraction orders that can contaminate zeroth order diffraction measurements. Applications of this type include SRAM, in-cell flash, and others.
Existing and proposed methods include the following: top view CD scanning electron microscopy (CD-SEM) that lacks the ability to provide details of 3D structures; cross-sectional SEM (X-SEM) that is destructive and cannot be used for inline metrology; CD-SAXS that has not yet been demonstrated to achieve high throughput capabilities required by the semiconductor industry; optical scatterometry CD metrology that is high throughput but is limited by SNR due to the limited light penetration into HAR structures; model-based infrared reflectometry (MBIR) (see, e.g., “Measuring deep-trench structures with model-based IR,” by Gostein et al., Solid State Technology, vol. 49, no. 3, Mar. 1, 2006, which is incorporated by reference as if fully set forth herein) that has been used for metrology of HAR DRAM structures but lacks the resolution provided by shorter wavelengths and has measurement spot sizes that are too large for semiconductor metrology; and atomic force microscopy (AFM) that cannot measure substantially high aspect ratio structures and has relatively low throughput. In summary, optical CD metrology is desirable but currently lacks the ability to measure the detailed profile of structures with micron scale depths and lateral dimensions in a relatively small spot (e.g., less than 50 microns, or even more preferably, less than 30 microns).
Accordingly, it would be advantageous to develop methods and systems for determining characteristics of structures formed on a wafer that do not have one or more of the disadvantages described above.